Solid phase reactions for the detection of an analyte in a sample have been significantly improved in the last several decades. In such reactions, the analyte is a member of a specific binding pair, typically either an antibody or an antigen to which the antibody specifically binds, although other binding pair types such as hormone/receptor and sugar/lectin are also used. The non-analyte member of the binding pair is attached to a solid substrate, and a sample suspected of containing the analyte is contacted with the substrate to which the specific binding pair member is attached. If the analyte is present, it is removed from solution by binding to the attached binding pair member. Detection of binding can be accomplished by numerous methods, such as competition for a labeled analyte, reaction of a second binding pair member that is labeled with the bound analyte (e.g., a sandwich immunoassay), or detection of some intrinsic measurable property of the analyte (such as enzyme activity).
In many cases the ease with which specific binding reactions can be carried out has led to the development of assays and test apparatuses designed for home use by an unskilled user or for use by trained non-professionals, such as police officers, under field conditions. When such apparatuses and methods are designed, care must be taken to avoid contamination and other types of improper use so that an accurate and reliable test result is obtained.
One such apparatus is described in U.S. Pat. No. 4,632,901 to Valkirs et al. The apparatus comprises a membrane or filter to which is bound an antibody, typically a monoclonal antibody, over a portion of the surface less than the total surface to which sample will eventually be applied. The apparatus further comprises an absorbent material in contact with the membrane or filter that acts to wick sample through the membrane or filter. Addition of the sample is followed by addition of a labeled antibody directed against the antigen being assayed (i.e., a sandwich assay) followed by a washing step to remove unbound labeled antibody. The absorbent material wicks unbound materials through the filter or membrane and allows reading of results from the upper surface of the membrane or filter to which the original antibody was bound. The reading can be either a visual color change or an instrumental reading, such as by a reflectance spectrophotometer.
U.S. Pat. No. 3,888,629 to Bagshawe describes a reaction cell for the performance of radioimmunoassay determinations and like saturation analysis reactions having supported within it a matrix pad of absorbent material capable of retaining the necessary reagents for the reaction and serving as a site in which the reaction totally occurs. A separable lower chamber is fitted to the lower end of the cell and contains absorbent material to abut the matrix pad and promote filtration through the pad after the reaction has taken place. An upper reservoir chamber fits to the upper end of the cell to contain liquid for passing through the matrix pad. The matrix pad will commonly contain prior to the reaction a predetermined amount of an antigen or antibody in freeze-dried condition.
However, these apparatuses exhibit a number of disadvantages, particularly in terms of the means available for detecting results more than simple positive results (i.e., quantitative or semi-quantitative results are generally not available) and the ability of the user to make a permanent record of the results using the test apparatus, a particularly important application for field testing.